Real runtime, not the number on the label — with depth of discharge and inverter losses taken off before the maths, because that is where most of the capacity goes.
The battery
V
%
How much of the battery you are willing to use. Draining lead-acid past 50% shortens its life sharply.
The load
%
85–90% through a decent inverter for AC loads. Use 95–100% for a DC load wired straight to the battery.
Runtime
—
—
Total capacity—
Usable capacity—
Load drawn from battery—
Current from battery—
Runtime at 100% discharge—
Discharge rate (C)—
Energy used per hour—
Runtime in days (24 h use)—
—
Embed this calculator on your site — free
Advertisement
Ad slot A
Why the label capacity is not the runtime
A 100 Ah 12 V battery holds 1,200 Wh on paper. Feed a 150 W load through an inverter and the naive answer is eight hours. The real answer is closer to five and a half — and the difference is not the battery underperforming, it is two deductions that always apply.
Depth of discharge. Lead-acid batteries are destroyed by deep cycling; taking a flooded or AGM battery below 50% state of charge cuts its cycle life dramatically, so only half the label capacity is genuinely available. Lithium iron phosphate tolerates 80–90% routinely, which is a large part of why it costs more and is still worth it.
Conversion losses. Any AC load runs through an inverter, and inverters are 85–92% efficient at reasonable load. Below about 15% of rated output they get noticeably worse, and their own idle draw — often 10–25 W — runs whether or not anything is plugged in.
The calculation
Runtime = (capacity in Wh × depth of discharge) ÷ (load in W ÷ efficiency). For the example above: 1,200 Wh × 80% = 960 Wh usable; 150 W ÷ 85% = 176 W actually drawn; 960 ÷ 176 = 5.4 hours.
Peukert's effect on lead-acid
Lead-acid capacity is quoted at a 20-hour discharge rate, and delivering it faster reduces what you actually get — pulling a 100 Ah battery at 50 A might yield only 65–75 Ah. Lithium is largely immune to this. The C-rate shown above tells you when it matters: below 0.1C the effect is negligible, above 0.3C it becomes significant on lead-acid.
To work the other way — from the runtime you need to the battery you should buy — multiply your load in watts by the hours required, divide by the efficiency, then divide by the depth of discharge you are willing to use. A 150 W load for 12 hours on lithium at 85% inverter efficiency needs 150 × 12 ÷ 0.85 ÷ 0.8 = about 2,650 Wh, which is roughly 220 Ah at 12 V.
Real loads are not constant
A fridge cycles on and off and may only run 35–45% of the time, so its average draw is well under its rated wattage. Motors and compressors also pull a surge several times their running current at startup, which the inverter must handle even though it barely affects runtime. If you are planning an off-grid system, budget from measured daily watt-hours rather than nameplate watts, and add 20% margin for cold weather — capacity falls with temperature, sharply so on lead-acid.
A 12 V 100 Ah battery holds 1,200 Wh. On lithium at 80% depth of discharge through an 85% efficient inverter, that is 960 Wh usable against 118 W actually drawn — roughly 8 hours. The same battery in lead-acid, limited to 50%, gives about 5 hours.
What depth of discharge should I use?
50% for flooded lead-acid and AGM, 80% for LiFePO4 in normal use, and up to 90% for lithium where cycle life matters less than capacity. Going deeper works but costs cycle life, and on lead-acid it costs a lot of it.
Do I need to allow for inverter efficiency?
Yes, for any AC load. A typical inverter is 85–90% efficient, so a 150 W appliance pulls about 175 W from the battery. DC loads wired straight to the battery avoid this almost entirely — use 95–100% for those.
Why does my battery last less than calculated?
The usual causes are age (capacity fades with cycles), cold temperature, a higher discharge rate than the battery was rated at, inverter idle draw, or loads that are higher in practice than on the label. Older lead-acid can be at 60–70% of its original capacity.
What is C-rate and does it matter?
C-rate is the discharge current as a fraction of capacity — 50 A from a 100 Ah battery is 0.5C. On lead-acid, rates above about 0.3C reduce usable capacity through Peukert's effect. Lithium is far less sensitive, but very high rates still cause heating and voltage sag.